A new phosphine-triggered tandem [3 + 4] annulation reaction between Morita-Baylis-Hillman carbonates and 1,4-diheteroatom dinucleophiles has been developed, which provides a facile synthetic method for saturated seven-membered 1,4-heterocycles such as 1,4-oxazepanes, 1,4-thiazepanes, and 1,4-diazepanes. Mechanistic investigation implies that this reaction takes place through a phosphine-catalyzed allylic alkylation followed by a general base-catalyzed intramolecular Michael cyclization.